| 摘要 |
PROBLEM TO BE SOLVED: To provide a clock switching device which can switch between two clock systems without interruption regardless of the clock select signal input timing.SOLUTION: In a clock switching device, a PLL is included which exerts oscillation frequency control based on a phase difference between an input signal based on an active system input clock and a feedback frequency-divided signal and thereby generates an oscillation clock. A phase difference period equivalent to a phase difference between a standby system input frequency-divided signal and the oscillation clock is detected, and assuming a point of time at which a duration equal to the phase difference period has elapsed after pre-system switchover frequency division of the oscillation clock started to be a post-system switchover start point, frequency division of the oscillation clock is started from that point of time to generate the feedback frequency-divided signal. Then, at a system switchover point, after reception of a select signal, which is determined depending on the phase difference period, system switchover is performed. |
